In a cellular communication network, the region served by the network is divided into a pattern of cells. Each cell has one or more antennas that communicate with mobile units (cellular telephones and/or data terminals) within its service area. The cell may be divided into sectors, each of which is typically served by a different antenna. In the context of the present patent application, the terms “cell” and “sector” are used interchangeably. The present invention deals with classifying signals received at various locations in the service region of the network according to the antenna transmitting the signals. (Such operations are also referred to herein as assigning the received signals to antennas, or associating the signals with antennas). In the context of a cellular network, it will be understood that classifying the signals according to the transmitting antenna has substantially the same meaning as classifying the signals according to cell or sector.
Each cell in a narrowband cellular network is assigned a fixed set of frequencies, also referred to as channels. Narrowband networks currently in use include primarily Time Division Multiple Access [TDMA] networks, such as Global System for Mobile [GSM] communication networks. In such networks, each traffic channel may be further divided into time slots, while control channels are transmitted continuously. (In contrast, Code Division Multiple Access [CDMA] networks assign a broad frequency band to each cell, and all operating channels are generally transmitted continuously.) In order to reduce interference between calls, the frequency channels in a narrowband cellular network are typically distributed among the different cells so that nearby cells use different channels.
The strengths of the signals reaching the mobile units from the antennas, and vice versa, are determined by the path loss of electromagnetic waves propagating between the antennas and the mobile unit locations. If the received signal level at a given location is too low, poor quality or coverage holes will result. On the other hand, during a call between a given mobile unit and a given antenna, the power of the traffic signals transmitted by the antenna on the channel or slot serving the mobile unit may be intentionally reduced, by a mechanism known as transmission power control (TPC). The object of this mechanism is to reduce interference with other calls on the same channel or adjacent channels in other nearby cells, while still providing the given mobile unit with a sufficient carrier/interference (C/I) ratio.
Because of the limited available spectrum, channel allocation generally involves tradeoffs between coverage of the service area and potential interference between different cells. If an inadequate number of channels are available at the location, calls will be blocked or dropped. On the other hand, if cells whose service areas overlap significantly use the same channels, mobile units in the overlap area will experience substantial interference. Therefore, an accurate map of cell service areas and signal strengths can be very useful in optimizing cellular network performance.
In planning cellular networks, path loss maps are typically used to locate and orient the antennas and determine channel allocations and transmission power levels. Such maps may be based on analysis of the topography and other characteristics of the network service area. In actual operation, however, the transmitted signals are subject to variable and unexpected attenuation, and the a priori path loss estimates are rarely completely accurate. To get a better picture of the actual distribution of cellular signals transmitted by the various antennas, cellular operators use drive tests, in which the actual antenna signals are measured at different locations by a test van driving through the service region.
At any location in the service region that is visited by the test van, it is likely to measure signals on multiple different frequency channels. The source of a signal on a given channel at a given location may be ambiguous, since as noted above, multiple cells may transmit on the same channel, and the received power is subject to TPC and fading, which may be difficult to predict. This problem can be resolved in part by assigning each cell a particular “color code,” i.e., a signal pattern that distinguishes its signals from those of other cells. In practice, however, the color code is not always available. Therefore, correctly associating each drive test measurement with a transmitting cell may be a difficult task.
U.S. Pat. Nos. 5,926,762 and 6,405,043, whose disclosures are incorporated herein by reference, describe methods for interference prediction and frequency planning for cellular networks based on drive test results. In order to avoid ambiguity in associating the signals measured in the drive test with the transmitting antennas, a single channel is transmitted from each cell site or sector, and all cells in the test area transmit on different channels. By transmitting from each cell on a single, unique channel, interference that might complicate the readings is eliminated, so that the cell from which any channel is transmitted is positively known. By implication, however, this test method can be carried out only when the cellular network is off-line, serving only the drive test and not serving network customers (or at best serving the customers on a very limited basis permitted by the restricted, single-channel spectrum allocated to each cell or sector.) There remains a need for a reliable method for processing drive test measurements that are gathered while the network is in normal operation.